The kraft pulping process is one of the major pulping processes in the pulp and paper industry. Spent liquor resulting from the kraft pulping process (black liquor or “BL”) contains various organic materials as well as inorganic salts, the deposition of which detracts from an efficient chemical recovery cycle. Inorganic pulping chemicals and energy are recovered by incinerating BL in a recovery boiler. For an efficient combustion in the recovery furnace, BL coming from the pulp digesters with relatively low solids concentration has to be evaporated and concentrated to at least 60% solids, typically in a multistage process (i.e., a multi-effect evaporator).
The alkaline pulping process differs from the kraft process in that no sodium sulfide is used in alkaline pulping, which results in less sodium sulfate in the spent liquor. In contrast, amounts of sodium, ammonium, magnesium, or calcium bisulfite are used in the sulfite process, resulting in high sulfate concentration in the spent liquor. The neutral sulfite semichemical (“NSSC”) process combines sodium sulfite and sodium carbonate. While the ratio between the inorganic, scale-forming components is different for these processes, the components are essentially the same.
Inorganic salt scaling in spent liquor evaporators and concentrators continues to be one of the most persistent problems encountered in the pulp and paper industry. Concentrated liquor contains calcium, sodium, carbonate, and sulfate ions at levels high enough to form scales that precipitate from solution and deposit on heated surfaces. The most important types of scale in evaporators are hard scale, such as calcium carbonate (CaCO3), and soft scale, such as burkeite (2(Na2SO4):Na2CO3). The solubility of both types of scale decreases as temperature increases, which causes the scales to adhere to heat transfer surfaces thus drastically reducing the overall efficiency of the evaporator (See Smith, J. B. & Hsieh, J. S., Preliminary investigation into factors affecting second critical solids black liquor scaling. TAPPI Pulping/Process, Prod. Qual. Conf., pp. 1 to 9, 2000 and Smith, J. B. & Hsieh, J. S., Evaluation of sodium salt scaling in a pilot falling film evaporator. TAPPI Pulping/Process, Prod. Qual. Conf., pp. 1013 to 1022, 2001; and Smith, J. B. et al., Quantifying burkeite scaling in a pilot falling film evaporator, TAPPI Pulping Conf., pp. 898 to 916, 2001).
Solubility of calcium carbonate in water is very low, whereas burkeite is soluble. Calcium carbonate deposits form extensively at many stages of the papermaking process. Control of calcium carbonate is a rather developed area outside evaporator applications. On the other hand, burkeite, which precipitates when total solids concentration reaches approximately 50%, represents a specific problem of evaporators and concentrators. While burkeite significantly affects productivity, neither monitoring methods nor chemical products exist for efficient burkeite control.
Affecting precipitation from a supersaturated solution of inorganic salts as water-soluble as burkeite is very difficult. (See U.S. Pat. Nos. 5,716,496; 5,647,955; 6,090,240). It is known though that sodium polyacrylate acts as a crystal-growth modifier for burkeite (See EP 0289312). Moreover, polyacrylic acids and methyl vinyl ether/maleic anhydride copolymers may act as inhibitors for soft scale, such as burkeite (See U.S. Pat. Nos. 4,255,309 and 4,263,092). Anionic/cationic polymer mixtures have also been suggested as scale control agents for evaporators. (See U.S. Pat. Nos. 5,254,286 and 5,407,583).
Generally, monitoring of inorganic scale is most efficiently achieved using quartz crystal microbalance (“QCM”) based technologies. Applicability of QCM-based instruments is determined, however, by sensor crystal stability under process conditions. Such instruments cannot be used under high temperature and/or high alkalinity conditions. This limitation makes the technology useless in digesters and evaporators. Besides a simple gravimetric technique and a non-quantitative characterization using Lasentec-FBRM®, a laboratory technique based on deposit accumulation on the heated surface was proposed for liquors with solid content higher than 55%. No methods have been proposed for use in spent liquor evaporators or concentrators under normal operating conditions.
There thus exists an ongoing need to develop alternative and more efficient methods of monitoring and inhibiting burkeite and other scale deposition in the pulp and paper industry. Such inhibition is of particular importance in pulp mill evaporators and concentrators.